The collection of biopotential signals, such as electrocardiographic (ECG), electromyographic (EMG), and electroencephalographic (EEG) signals, is commonly used in minimally invasive techniques for obtaining diagnostic and patient monitoring data. These techniques are performed by placing a plurality of biomedical electrodes in electrical contact with the patient's skin. A patient connection system includes the plurality of electrodes, arranged in different standard configurations depending on the specific biopotential signals to be collected, and lead wires attached to the electrodes. The electrodes sense the electrical signals generated by the patient's heart, muscles, or neural pathways. For example, in ECG, electrical potentials generated in the heart are collected by a system of three, five, or ten electrodes.
Biopotential electrodes are typically single-use and disposable. They rely on a layer of conductive and adhesive gel to both create the electrical connection with the patient's skin and removably affix the electrode to the patient. The adhesive properties of these electrodes deteriorate over the duration of use thereby diminishing the conductive properties as well. This deterioration in conductive properties occurs in two general situations: first by compression and flexion due to the motion of the electrode site if the patient is active, or secondly, when the electrodes have been attached to the patient for an extended period of time, as is found in many long-term hospital and critical care situations.
Therefore, there are clinical advantages to a system that monitors the quality of the connection of electrodes to a patient's skin as, for example, that shown in Simon et al. U.S. Pat. No. 4,577,639. Furthermore, a system that detects a poor connection or disconnected electrode and sends a signal to clinicians to warn them to replace the electrode is also desired. Additionally, or alternatively, the signal could activate a lead-switching network, to select a proper configuration from those electrodes having good connections and maintain a valid measurement with this new configuration, as is also shown in Simon et al.
Most electrode connection quality measurement techniques currently in use measure the resistance or the impedance of the electrode connection. Electrode connection impedance is a composite measurement of many sources of impedance. These include minor contributors such as the impedance of the electrode itself (˜100 Ohms), the patient's internal impedance (˜100 Ohms), and the impedance of the conductive material in the electrode (˜1 KOhm). But the electrode impedance is primarily the electrode-skin interface impedance. This is usually stated to be between 15KΩ and 1MΩ. See Med Instr. Application & Design, Webster, Ed et al. (1998) p. 198.
The epidermal layer of the skin behaves electrically as a parallel RC circuit, and therefore the impedance of the electrode-skin interface is frequency dependent. This is characterized by an epidermal impedance that ranges from approximately 200 KOhms at 1 Hz to 200 Ohms at 1 MHz. (Webster et al.) As the connection between the electrode and/or the skin deteriorates, and should the electrode detach, the impedance of the electrode-skin interface increases.
Electrode connection quality measurement devices are combined with lead switching technology to provide a continuous high quality biopotential measurement. A lead-switching network selects the proper combination of well connected electrodes to produce the desired biopotential signal or signals. An example of such a system is disclosed in Simon '639. An additional example of the electrode connection quality measurement is depicted in FIG. 1 of Marriott U.S. Pat. No. 5,020,541.
Two approaches have developed in the art to determine electrode connection quality. Each approach has its advantages and its limitations. One approach, shown in Simon '639, teaches the injection of a constant DC current to the electrodes. This current will create a DC bias voltage across the electrode-skin interface which is directly proportional to the resistance of the electrode connection. A threshold voltage is set that is indicative of a disconnected electrode and once this threshold is met a “leads off” condition is indicated to the clinician via an alarm or a visual display. Additionally, the lead switching network monitors which electrodes are still well connected and determines the optimal combination of the remaining electrodes to produce a quality ECG signal. This determination is made by referring to a predetermined set of lead switching alternatives or preset optimal combinations based upon which ECG leads are currently in use and which ECG electrode has been disconnected.
The advantage of using a DC current is that no additional frequency component is injected on the patient. Additional frequency components can interfere with the monitoring of other physiological signals, such as EEG and EMG. The disadvantage of the DC method is that the varying DC bias or offset voltage, that provides the measurement of electrode quality, corrupts the biopotential signal and must later be taken out or compensated for to provide an accurate measurement of the biopotential signal. Also, the voltage drop resulting from the DC current may be hard to distinguish from other DC offsets that are present in ECG measurement. Another disadvantage of the DC method is that a DC current will only produce a voltage correlated to the resistance of the electrode connection. The electrode, however, is not purely resistive and to provide an accurate analysis of the electrode connection quality other frequency dependent properties, such as capacitance, must also be taken into account.
The approach of utilizing an AC signal for the on-off determination of electrode connection is taught by Morgan in U.S. Pat. No. 4,619,265. This approach is similar in method to the DC approach, except that an AC signal is injected and the impedance of the electrode is measured instead of the resistance. The Marriott '541 patent, noted above, depicts such use of an AC signal for an ECG electrode quality determination system. Marriott '541 discloses the injection of two out of phase AC carrier signals. One signal is sent to a reference lead and the other signal is sent to a plurality of collection leads. The different combinations of ECG electrode signals are supplied to differential amplifiers wherein the amplitudes of two AC carrier signals are compared to determine a differential voltage measurement that corresponds to the impedance of the electrode interface for a given ECG lead. This measurement can be analyzed by downstream components to detect and indicate electrode connection quality or a leads off condition.
The advantage of utilizing AC signals is that the impedance of the electrode connection can be measured to provide a more accurate picture of the entire connection over that provided by the DC method that measures only resistance. The disadvantage of utilizing AC signals is that they inject a frequency component onto the patient. This can interfere with the concurrent measurement of other physiological parameters of the patient by other pieces of monitoring equipment, especially equipment such as EEG and EMG equipment.
While the foregoing describes the collection of biopotential signals from a patient, the electrical quality of a patient-electrode interface is also important when electrical energy is applied to a patient, as by a defibrillator.